Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes, for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons; that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets (and pole pieces) have been tried in an attempt to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are typically arranged in a sequence of alternating magnetization, either parallel to, or anti-parallel to, the direction of the electron flow. The magnets (and pole pieces) are usually annular in shape and their axes are aligned with the path of the electron beam. Pole pieces constructed of ferromagnetic material such as electrolytic iron are often placed between the magnets and provide a path through which magnetic flux from the magnets may be directed into the working space along the axis of the traveling wave tube in order to influence the beam in the desired manner. The patent to Clarke, U.S. Pat. No. 4,731,598, issued Mar. 15, 1988, illustrates typical prior art, periodic permanent magnetic (PPM) structures.
One of the critical problems confronting those who develop magnetic structures used to contain or manipulate beams of charged particles has been how to more efficiently utilize the permanent magnet materials which make up the structure(s). Some specific problems include: how to maximize the strength of the magnetic field along the path of the charged particle beam without significantly increasing the mass of the magnetic structure; how to improve performance (e.g., output power); and how to increase the useful life of the TWTs. The present invention addresses these problems, and others.
The above-noted problems were also addressed in the co-pending patent application Ser. No. 213 970, filed July 1, 1988, now U.S. Pat. No. 4,831,351, which is incorporated by reference herein. In this co-pending application there is disclosed a periodic permanent magnet (PPM) structure that comprises a series of hollow spherical flux sources (HSFS) or "magic spheres" placed tangent to each other in pearl string fashion. Axial bore holes through the magnetic poles of the spheres are coaxially aligned to form with spherical central cavities a continuous channel or path through which a beam of charged particles will travel. In any given magic sphere the magnetic field orientation in the axial bore hole is the reverse of that in the central cavity. Thus, the desirable characteristic of alternating magnetization in a PPM stack is fully realized in a string of coaxially aligned magic spheres. This HSFS PPM stack offers focusing fields of about 10 kOe. This is substantially greater than the approximately 6 kOe theretofore obtained in prior art PPM structures.